The invention relates to a system for at least far-reaching compensation of directional and positional fluctuations in light produced by a laser, in particular for micro lithographic illuminating devices. The invention also relates to a device therefore.
It is generally known to influence and vary light produced by a laser, in particular with reference to rotation and focusing. Reference may be made for this purpose to EP 293823 B1 and U.S. Pat. No. 4,703,166, for example. Both patent specifications relate to an active focusing sensor device, the aim being to tune the wavelength of a laser beam in order to find the magnitude of the spacing of the objective from a wafer. Active control is performed for this purpose by means of appropriate rotation of mirrors. At the same time, the aim in this case is also to compensate vibration al movements of a positioning table.
Laser-operated illuminating systems such as are used, for example, in micro lithographic projection exposure machines react sensitively to positional and directional fluctuations in the laser radiation at the input to the system. In particular, this exerts a negative influence on the uniformity of the light distribution and the centered beam angular characteristic (telecentrism). Again, it is not always possible to avoid positional and directional fluctuations while observing tolerances which are as small as possible. Thus, for example, changes occur simply from heating of the laser during operation.
It is therefore the object of the present invention to compensate as far as possible positional and directional fluctuations in light produced by a laser, in particular for micro lithographic illuminating devices which exert a negative influence on the uniformity of the light distribution and the characteristic of the centered beam angle.
This object is achieved according to the invention in systematic terms by means of the features named in claim 1.
A device for this purpose is exhibited in claim 3.
According to the invention, the laser beam produced by a laser is partitioned into two partial beams at a beam splitter between the laser and the illuminating system or the illuminating device, a first partial beam being guided directly onto an illuminating reference surface. The second partial beam is guided via a detour having at least two beam-deflecting devices, before subsequently also being fed to the illuminating reference surface after renewed traversal of the beam splitter.
The measure according to the invention renders it possible to balance fluctuations in the laser with reference to its position, and thus to eliminate their action on the illuminating system.
At the same time, the system according to the invention and the device therefore can also be used to achieve a compensation of directional fluctuations in the laser. Although a slight beam offset occurs in this case, in practice the beam offset is less than 1% of the beam width. It is important that the proposed system delivers stabilization of the centroid beam position and the centroid beam direction.
Since the beam splitter causes a plurality of round-trip passes by the partial beam guided via the beam-deflecting device, it can advantageously be provided for a more uniform light distribution that as regards the partition of the laser beam the splitting ratio of the first partial beam to the second partial beam is 33.3/66.7.
A beam splitter cube can advantageously be used as beam splitter, because this avoids a beam offset which would be produced by a beam splitter plate itself. The beam splitter cube has, of course, to be tuned to the wavelength employed.
Path-folding mirrors can be used in a simple way as beam-deflecting devices.